The present invention relates generally to optical gratings and more particularly to a method for making such gratings having an interlayer of differing refractive indexes.
Optical technology is progressing rapidly. Growing needs, particularly in the telecommunications industry, are driving this progress and there is currently a major impetus to improve existing optical systems and to develop new ones. Unfortunately, several major components still are not completely meeting manufacturing yield, field reliability, and operating capacity requirements. These failings have resulted in high costs in existing systems and are limiting the adoption of future systems. One such component is the optical grating, and particularly the fiber Bragg grating.
The common fiber Bragg grating is a periodic perturbation in the refractive index which runs lengthwise in a fiber waveguide""s core. Based on the grating period, a Bragg grating reflects light within a narrow spectral band and transmits all other wavelengths which are present but outside that band. This makes Bragg gratings useful for light signal redirection, and one application where they arc now being widely used is wavelength division multiplexing (WDM) in optical communications networks.
The typical fiber Bragg grating today is a germanium-doped optical fiber that has been exposed to ultraviolet (UV) light under a phase shift mask or grating pattern. The unmasked doped sections undergo a permanent change to a slightly higher refractive index after such exposure, resulting in an interlayer or a grating having two alternating different refractive indexes. This permits characteristic and useful partial reflection to then occur when a laser beam transmits through each interlayer. The reflected beam portions form a constructive interference pattern if the period of the exposed grating meets the condition:
2*xcex9*ncff=xcex
where xcex9 is the grating spacing, ncff is the relative index of refraction between the unchanged and the changed indexes, and xcex is the laser light wavelength.
FIG. 1 (background art) shows the structure of a conventional fiber Bragg grating 1 according to the prior art. A grating region 2 includes an interlayer 3 having two periodically alternating different refractive indexes. As a laser beam 4 passes through the interlayer 3 partial reflection occurs, in the characteristic manner described above, forming a reflected beam 5 and a passed beam 6. The reflected beam 5 thus produced will include a narrow range of wavelengths. For example, if the reflected beam 5 is that being worked with in an application, this separated narrow band of wavelengths may carry data which has been superimposed by modulation. The reflected beam 5 is stylistically shown in FIG. 1 as a plurality of parts with incidence angles purposely skewed to distinguish the reflected beam 5 from the laser beam 4.
Unfortunately, as already noted, conventional fiber Bragg gratings and the processes used to make them have a number of problems which it is desirable to overcome. For example, the fibers usually have to be exposed one-by-one, severely limiting mass-production. Specialized handling during manufacturing is generally necessary because the dosage of the UV exposure detemimes the quality of the grating produced. The orientation of the fiber is also critical, and best results are achieved when the fiber is oriented in exactly the same direction as the phase shift mask. The desired period of the Bragg grating will be deviated from if the fiber is not precisely aligned and accomplishing this, in turn, introduces mechanical problems. The way that the fiber work piece is held during manufacturing may produce stresses that can cause birefringes to form in the fiber and reduce the efficiency of the end product grating.
Once in use, conventional fiber Bragg gratings may also require special handling. The thermal expansion coefficient of the base optical fiber is often significant enough that changing environmental conditions can cause the fiber to either expand or shrink to the extent that the period of the grating and its center wavelength shift.
Accordingly, what is needed is an improved process for fabricating optical gratings, and particularly for the subset of optical gratings known as Bragg gratings.
Accordingly, it is an object of the present invention to provide improved processes to manufacture optical gratings.
And another object of the invention is to provide improved processes to manufacture Bragg type optical gratings.
Briefly, one preferred embodiment of the present invention is a method for fabricating an optical grating. The method includes providing a substrate, a layer of transmissive material, a first reflector below the layer of transmissive material, a grating region in the layer of transmissive material, and providing a second reflector above the layer of transmissive material.
An advantage of the present invention is that it provides improved processes to manufacture a variety of optical gratings, particularly including Bragg type optical gratings.
Another advantage of the invention is that it does not use fiber based media, thus eliminating a number of disadvantages in both fiber based manufacturing processes and fiber media based grating products.
Another advantage of the invention is that it may employ already well known and widely used manufacturing processes and materials, adopted from conventional electronic semiconductor integrated circuit (IC) and micro electromechanical system (MEMS) manufacturing. Highly desirable attributes of such processes may thus be imparted to the inventive processes and the products produced there with, including mass automated manufacturing, rigorous quality control, high yields, and low cost.
Another advantage of the invention is that it permits easy, very high integration with other products of conventional IC and MEMS manufacturing processes.
And another advantage of the invention is that it permits the manufacture of multidimensional gratings.
These and other objects and advantages of the present invention will become clear to those skilled in the art in view of the description of the best presently known mode of carrying out the invention and the industrial applicability of the preferred embodiment as described herein and as illustrated in the several figures of the drawings.